172 A. A. C. ELIOT MERLIN ON THE DRONE-FLY's PROBOSCIS. 



Formerly, the Podura-scale was most generally and frequently 

 used for testing the class of lenses referred to. The mode of 

 employing it for this purpose was described by the late Richard 

 Beck in a paper read before the London Microscopical Society on 

 .March 12th, 18G2. This paper is accompanied by a beautiful 

 plate illustrating the varying appearances of the markings. 

 There can be no doubt that this object is a valuable and effective 

 test for centring and colour correction, but, even apart from the 

 fact that the right kind of Podura has apparently became extinct 

 or excessively wary, and that consequently it is at present difficult 

 to procure a good scale in optical contact with the cover-glass, as 

 it should be to insure satisfactory results, here again considerable 

 practical experience is required on the part of the manipulator. 



It is suggested that in a balsam mount of the cut suctorial 

 tubes of the common Drone-fly's proboscis, the structure of which 

 is precisely similar to, although very much finer than, that of the 

 Blow-fly, we possess a test which should enable even persons of 

 small experience to form a trustworthy opinion regarding the 

 quality of the ordinary cheap ^ in. and ^ in. objectives usually 

 supplied with small histological microscopes. For test purposes, 

 at least, a J solid axial cone should be employed, and if with such 

 illumination, and suitable tube length for cover correction, the 

 " arches " and intervening fibres appear well separated and neatly, 

 although probably somewhat faintly, defined, the lens may be 

 safely considered a good one ; for under the specified conditions a 

 really poor objective would fail to show any distinct structure, 

 and would break down utterly and unmistakably. 



It has been found in practice that Powell's old achromatic 



in., of measured N.A. '79 and working aperture -6, with 12 

 eye-piece, will completely resolve the structure referred to, while 

 the Zeiss apochromatic h in., measured N.A. '70, W.A. G, does 

 so beautifully with 40 eve-piece. The Leitz semi-apochromatic 

 : in., ~ in., and i in. all show this delicate object clearly with 

 very large illuminating cones, so that any cheap modern | in. 

 may be reasonably expected and required to do so. 



Jow n. Quekett Microscopical Club, So: 2, Vol. IX., No. 56, April 1905. 



